1. Field of the Invention
The invention relates to radar systems generally and, more particularly, to CFAR video signal processing therein.
2. Description of the Prior Art
The term, CFAR (constant false alarm rate), is well understood in the radar arts.
The concept of "false alarms" relates to undesired echoes from rain and other hydrometeoric phenomena, clutter and back scatter signals from other radiating sources which, in some cases, can exceed the bona fide target signal level, at least over the short term. Often, these undesired signals exceed receiver noise level and may completely obliterate a radar display or may overload signal processing computers which make the yes-no decisions as to which signals are bona fide echoes or targets of interest.
The textbook radar handbook by Merrill I. Skolnik (McGraw-Hill, 1970) discusses CFAR considerations in Chapter 5, devoted to receivers. In particular, CFAR considerations are discussed at Section 5.8 in that chapter, and the logarithmic video detector receiver technique is discussed in Section 5.9 .
The use of a radar receiver with logarithmic detector is known to be useful in maintaining a constant false alarm rate in the presence of variable intensities of noise, rain clutter, sea return, etc.
The patent literature and other technical publications contain many examples of video signal processing in order to enhance signal-to-noise ratio in difficult environments.
In the special case of surface surveillance by radar, such as in airport surface detection and tracking of aircraft or other vehicles, a special set of problems arise. Among these are the large extent of the target return signal compared to the size of the radar range resolution cell, there being a 5-1 ratio in a typical situation involving a large modern airliner on a runway. In addition, the range intervals adjacent to the runway are likely to contain large amplitude ground clutter, due to off-runway terrain. Still further, a region of relatively low amplitude ground clutter generally exists in small range regions immediately adjacent to the target return signal. This is the region of runway back scatter.
The above recited special conditions rule out conventional types of prior art CFAR processing. Log-FTC is inapplicable, due to the waveform differentiation process used, only the leading edge of the target return waveform being retained in the processor output. Preservation of target return shape on an operational display for a successful airport ground surveillance radar system is a necessity.
The so-called delay line differentiator (DLD), pulse-length discriminators (PLD), and high pass side lobe reduction filter circuits (SRF) also differentiate the target return waveform and therefore are not suitable for this application. A paper by R. J. Evans and E. F. Thomas, entitled, Optimal Resolution of Rectangular Pulses and Noise, IEEE Transactions, AES-11, No. 3, May, 1975, pages 372-379, describe the prior art SRF technique.
The technique of cell averaging (or mean level detection) is a form of CFAR processing described by V. G. Hansen and H. R. Ward, in their paper, Detection Performance of the Cell Averaging LOG/CFAR Receiver, IEEE Transactions, AES-8, No. 5, September, 1972, pages 648-652. Such a technique is also inapplicable for the particular requirement aforementioned, since a set of reference radar resolution cells which are free from ground clutter cannot be guaranteed in the ground surveillance environment. Off-runway ground clutter would tend to capture this form of CFAR and thereby severely degrade target detectability.
The manner in which the present invention deals with the disadvantages of the prior art to provide a novel form of CFAR, particularly adaptable to the surface surveillance problem, will be evident as this description proceeds.